Although increases in plasma choline cause some increase in brain tissue choline, there is no increase in acetylcholine levels or in acetylcholine turnover rate in any of the brain areas studied. Indeed, increased plasma choline reduced the turnover rate of acetylcholine in the hippocampus demonstrating that increasing the availability of choline does not increase the rate of acetylcholine synthesis. Subcutaneously injected apomorphine appears to have a biphasic effect on the turnover rate of acetylcholine. Lower doses appear to reduce the turnover rate of acetylcholine in the striatum whereas at higher doses the turnover rate returns to normal.